When changing shock oils, it is best to disassemble the shock and inspect it's gaskets and retainers for problems, unless it is a new or freshly rebuilt shock.

In the event it is a fresh shock first remove the cap and remove the piston from the shaft. Wad up a paper towel and shove it into the shock, picking up the oil. Flush with denatured alcohol, then use a clean towel to dry it and re-assemble. Fill the shock with the oil of choice and work the air bubbles out. Let it sit while the bubbles work their way out and re tighten the cap.

If the shock shaft is able to bottom out on the shock body (it should rebound about half way out) then this is a good shock.

If you cannot fully compress the shock then it has too much oil and will need to be bled off, until the earlier results are achieved.

If the shock bottoms out and does not rebound then there is not enough oil and will need to be filled more. Remove the cap and add oil. Let it sit because there will be a large amount of bubbles in the oil once again, then try it again. It is always easier to start out with too much than not enough, but it makes a bit more of a mess.